A. The essence is two reagents that produce a color, such as a colorless solution that chars with application of heat or darkens with addition of lemon juice or urine, says Dr. Joe Schwarcz in The Genie in the Bottle. Lemon juice was apparently known during the French Revolution, and urine has allowed "many a message to be sent from one penitentiary inmate to another," Schwarcz says.

Writings in a colorless solution of phenolphthalein will turn pink when exposed to ammonia vapors, as used during World War II by German spy George Dasch, who was caught in the U.S. with a suspicious number of plain white handkerchiefs. The FBI solved the chemistry puzzle, revealing Dasch's contacts on the hankies.

More spectacular is Prussian blue, formed when ferric sulfate and potassium ferrocyanide react. Generally, messages in ferric sulfate solution are revealed when sprayed with ferrocyanide, as done by German spy George Vaux on his socks and cloth buttons. Sodium sulfide will work as well.

Even in this hi-tech age of clandestine encryptions, secret inks may not be passé, says Dr. Joe.

Egyptian authorities reportedly arrested two men with secret markings on ladies' undergarments, which they were seen passing back and forth. "If the case comes to trial, its outcome may well hinge on the chemistry of color-changing underwear."

Q. If my German Shepherd's sense of smell is 10,000 to 100,000 times more powerful than mine, why doesn't he keel over if he's in the laundry room with me and I open a bottle of bleach?– Sgt. Preston

A. It's true a dog has a much keener sense of smell, with 18-150 square cm. of olfactory area (nasal mucosa) compared to 3-4 in humans, says University of Minnesota veterinary and biomedical scientist Alvin Beitz. So a dog can detect far fewer molecules in the air. But it's the irritating properties of the bleach and not the distinct chemical smell that would cause your German Shepherd– or you– to keel over if exposed to too much for too long.

"It turns out that bleach activates pain nerve fibers in the nose. Since a dog and a human have about the same number of pain fibers in this area, the two respond similarly to irritating odors like bleach or ammonia," Beitz says.

Q.After Earth, which is the best-mapped planet in the solar system? –G. Galilei

A. Forget the Earth. It isn't #1 at all. If by mapping you mean "topographic" surface mapping, then Venus takes the prize, says research scientist Larry A. Lebofsky of the University of Arizona's Lunar and Planetary Laboratory.

Thanks to the robotic spacecraft Magellan, which began orbiting Venus in 1990, more than 98 percent of that planet's surface is mapped, revealing details as small as a football field across. In spite of Venus's 40-mile-thick sulfurous clouds, Magellan penetrated them, showing enormous volcanoes, lava flows, craters.

But if you look at the 1240 shaded-relief, geologic, topographic, photographic, digital and hard-copy planetary maps published by the U.S. Geological Survey, 696 are of Mars, only 33 of Venus, says Research Geologist Mary Chapman of the U.S. Geological Survey. The USGS Regional Planetary Image Facility confirms that most Mars data came from the two Viking Orbiter missions and that the percentage of Mars- mapping is even higher than Venus' 98 percent.

Earth, on the other hand, has 71% of its surface covered by water, with large portions of the ocean floor remaining unmapped.

"Both Venus and Mars beat out Earth mapping," says Lebofsky, "since we still have submarines running into underwater mountains!"

Q.The elephant's heart was in excellent condition, her other vital organs were sound, she was free of infection and disease. Yet she was dying, starving to death while surrounded by food. Why? –R. O. T. Jungle

A. Because she had worn out her last set of teeth, say Gil Brum et al. in Biology: Exploring Life. The coarse plant diet of the pachyderm grinds down its molars, which are periodically replaced. But after six sets, by about age 60, the party's over, and for many old elephants that can no longer chew their food, starvation becomes inevitable.